1. Field of the Invention
The invention relates to a hybrid electric vehicle powertrain and a method for avoiding a zero motor torque to attenuate torsional disturbances.
2. Background Art
A hybrid electric vehicle powertrain has two power sources comprising an engine and an electric traction motor in separate power flow paths. The power flow paths are defined by gearing, an output gear element being mechanically coupled to vehicle traction wheels. Typically, the power output element is connected through a drive shaft to a differential-and-axle assembly for the vehicle traction wheels. A powertrain of this type is disclosed, for example, in U.S. Pat. No. 7,273,439, which is assigned to the assignee of the present invention.
The powertrain disclosed in the '439 patent is a so-called power-split hybrid electric vehicle powertrain wherein an engine is drivably coupled to a torque input element of a planetary gear unit and a generator is coupled to a second element of the planetary gear unit. A third element of the planetary gear unit distributes torque to torque transfer gearing, which defines the torque flow path from both the motor and the engine as the two power sources drive the vehicle traction wheels.
A transmission controller is used to control generator speed, thereby effecting a division of engine power between the two power flow paths. This implies that the engine speed can be isolated from the vehicle speed, whereby the powertrain can function in a manner similar to the function of a continuously variable transmission where vehicle speed changes do not depend on engine speed changes. The engine and the motor, which is electrically coupled to the generator, function together seamlessly as they deliver torque through the gearing to the vehicle traction wheels.
The transmission controller may allow the motor torque to decrease to a zero value or a near-zero value as it controls torque delivery to the traction wheels in response to changing vehicle operating variables. If the motor torque becomes zero or approaches zero, torque pulsations from the engine due, for example, to engine combustion forces, may cause the unloaded motor and the gearing drivably connected to the motor to become subjected to torsional disturbances. These disturbances may cause what is commonly referred to as “gear rattle”.